Inhibiting bacterial population growth and altering the relative ratios of different bacterial species in a mixture finds application in a wide range of industries and settings, for example for treatment of waterways, drinking water or in other environmental settings. Application is also found in altering bacteria in humans and non-human animals, eg, livestock, for reducing pathogenic infections or for re-balancing gut or oral microbiota. Recently, there has been interest in analysing the relative proportions of gut bacteria in humans with differing body mass or obesity profiles, or in investigating possible bacterial influence in disease contexts such as Crohn's disease.
Although bacterial innate immune mechanisms against phage abound, an extensively documented bacterial adaptive immune system is the CRISPR/Cas system. Engineered CRISPR/Cas systems have been used for precise modification of nucleic acid in various types of prokaryotic and eukaryotic cells, ranging from bacterial to animal and plant cells (eg, see Jiang W et al (2013)). Prokaryotes, such as bacteria and archaea, encode adaptive immune systems, called CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR associated), to provide resistance against mobile invaders, such as viruses (eg, bacteriophage) and plasmids. Reference is made to Seed et al (2013), which explains that bacteriophages (or phages) are the most abundant biological entities on earth, and are estimated to outnumber their bacterial prey by tenfold. The constant threat of phage predation has led to the evolution of a broad range of bacterial immunity mechanisms that in turn result in the evolution of diverse phage immune evasion strategies, leading to a dynamic co-evolutionary arms race.
Host immunity is based on incorporation of invader DNA sequences in a memory locus (CRISPR array), the formation of guide RNAs from this locus, and the degradation of cognate invader DNA (protospacer) situated adjacent a protospacer adjacent motif (PAM). See, for example WO2010/075424. The host CRISPR array comprises various elements: a leader (including a promoter) immediately 5′ of one or more repeat-spacer-repeat units where the repeats are identical and the spacers differ. By acquiring spacer sequence from invading virus or plasmid nucleic acid, the host defence system is able to incorporate new spacers into the CRISPR array (each spacer flanked by repeats) to act as a memory to tackle future invasion by the virus or plasmid. It has been observed that recently-acquired spacers tend to be inserted into the host array directly after the leader.
Reference is made to Heler et al (2014), which explains that CRISPR loci and their associated genes (Cas) confer bacteria and archaea with adaptive immunity against phages and other invading genetic elements. A fundamental requirement of any immune system is the ability to build a memory of past infections in order to deal more efficiently with recurrent infections. The adaptive feature of CRISPR-Cas immune systems relies on their ability to memorize DNA sequences of invading molecules and integrate them in between the repetitive sequences of the CRISPR array in the form of ‘spacers’. The transcription of a spacer generates a small antisense RNA that is used by RNA-guided Cas nucleases to cleave the invading nucleic acid in order to protect the cell from infection. The acquisition of new spacers allows the CRISPR-Cas immune system to rapidly adapt against new threats and is therefore termed ‘adaptation’ (ie, vector sequence spacer acquisition).
Seed et al (2013) reported a remarkable turn of events, in which a phage-encoded CRISPR/Cas system was used to counteract a phage inhibitory chromosomal island of the bacterial host. A successful lytic infection by the phage reportedly was dependent on sequence identity between CRISPR spacers and the target chromosomal island. In the absence of such targeting, the phage-encoded CRISPR/Cas system could acquire new spacers to evolve rapidly and ensure effective targeting of the chromosomal island to restore phage replication. Bondy-Denomy et al (2012) describe the early observed examples of genes that mediate the inhibition of a CRISPR/Cas system. Five distinct ‘anti-CRISPR’ genes were found in the genomes of bacteriophages infecting Pseudomonas aeruginosa. Mutation of the anti-CRISPR gene of a phage rendered it unable to infect bacteria with a functional CRISPR/Cas system, and the addition of the same gene to the genome of a CRISPR/Cas-targeted phage allowed it to evade the CRISPR/Cas system.
Immature RNAs are transcribed from CRISPR arrays and are subsequently matured to form crRNAs. Some CRISPR/Cas systems also comprise sequences encoding trans-activating RNAs (tracrRNAs) that are able to hybridise to repeats in the immature crRNAs to form pre-crRNAs, whereby further processing produces mature, or crRNAs. The architecture of cRNAs varies according to the type (Type I, II or III) CRISPR/Cas system involved.
CRISPR-associated (cas) genes are often associated with CRISPR arrays. Extensive comparative genomics have identified many different cas genes; an initial analysis of 40 bacterial and archaeal genomes suggested that there may be 45 cas gene families, with only two genes, cas1 and cas2, universally present. Cas1 and Cas2 are believed to be essential for new spacer acquisition into arrays, thus are important in mechanisms of developing resistance to invader nucleic acid from phage or plasmids. Nuñez et al (2015) reportedly demonstrated the Cas1-Cas2 complex to be the minimal machinery that catalyses spacer DNA acquisition and apparently explain the significance of CRISPR repeats in providing sequence and structural specificity for Cas1-Cas2-mediated adaptive immunity.
CRISPR/Cas systems also include sequences expressing nucleases (eg, Cas9) for cutting invader nucleic acid adjacent cognate recognition motifs (PAMs) in invader nucleotide sequences. PAM recognition of nucleases is specific to each type of Cas nuclease. The PAMs in the invader sequences may lie immediately 3′ of a protospacer sequence, with nucleases typically cutting 3-4 nucleotides upstream of (5′ of) the PAM. The conservation of the PAM sequence differs between CRISPR-Cas systems and appears to be evolutionarily linked to cas1 and the leader sequence. Fineran et al (2014) observed that Invaders can escape type I-E CRISPR-Cas immunity in Escherichia coli K12 by making point mutations in a region (the “seed region”) of the protospacer or its adjacent PAM, but hosts quickly restore immunity by integrating new spacers in a positive-feedback process involving acquisition (“priming”). To date, the PAM has been well characterized in a number of type I and type II systems and the effect of mutations in the protospacer has been documented (see references 5, 14, 23, 46, 47 in Fineran et al (2014)). Fineran et al (2014) concluded that their results demonstrated the critical role of the PAM and the seed sequence, in agreement with previous work.
Semenova et al (2011) investigated the role of the seed sequence and concluded that that in the case of Escherichia coli subtype CRISPR/Cas system, the requirements for crRNA matching are strict for the seed region immediately following the PAM. They observed that mutations in the seed region abolish CRISPR/Cas mediated immunity by reducing the binding affinity of the crRNA-guided Cascade complex to protospacer DNA.
The stages of CRISPR immunity for each of the three major types of adaptive immunity are as follows:—
(1) Acquisition begins by recognition of invading DNA by Cas1 and Cas2 and cleavage of a protospacer;
(2) A protospacer sequence is ligated to the direct repeat adjacent to the leader sequence; and
(3) Single strand extension repairs the CRISPR and duplicates the direct repeat.
The crRNA processing and interference stages occur differently in each of the three major types of CRISPR systems. The primary CRISPR transcript is cleaved by Cas to produce crRNAs. In type I systems Cas6e/Cas6f cleave at the junction of ssRNA and dsRNA formed by hairpin loops in the direct repeat. Type II systems use a trans-activating (tracr) RNA to form dsRNA, which is cleaved by Cas9 and RNaseIII. Type III systems use a Cas6 homolog that does not require hairpin loops in the direct repeat for cleavage. In type II and type III systems secondary trimming is performed at either the 5′ or 3′ end to produce mature crRNAs. Mature crRNAs associate with Cas proteins to form interference complexes. In type I and type II systems, base-pairing between the crRNA and the PAM causes degradation of invading DNA. Type III systems do not require a PAM for successful degradation and in type III-A systems base-pairing occurs between the crRNA and mRNA rather than the DNA, targeted by type III-B systems.